Solar Panels on Flat Roofs and Rear Extensions

Yes, flat roofs can take solar panels, and they often work well. Panels go on mounting frames that tilt them towards the sun, held in place either by weight (ballast) or by fixings into the roof. The things that decide whether it works are the condition of the roof covering, the strength of the structure, the planning limits on height, and how much shade falls on the roof.

Across London, flat roofs are everywhere: rear extensions, dormers, garages and the main roofs of some houses and flats. This guide covers what matters for each.

Key points

• Panels on a flat roof sit on frames at a low tilt, facing south or arranged east and west in pairs.

• Frames are held down by ballast, by fixings, or by a combination, designed for the wind and the roof structure.

• Under permitted development, panels on a flat roof must not stand more than 0.6 metres above the highest part of the roof, not counting chimneys.

• On article 2(3) land, which includes conservation areas, you must apply to the council before installing on a flat roof.

• The roof covering should have plenty of life left. Replacing it later means taking the panels off.

• Rows need spacing so they don’t shade each other.

How panels are mounted on a flat roof

On a pitched roof, panels are fixed close to the slope. On a flat roof they need a frame to tilt them towards the sun and shed rain. There are two main ways to hold the frames down.

Ballasted systems. The frames rest on the roof, on protective pads, and are held in place by weights such as concrete blocks. There are no holes in the roof covering, which keeps the waterproofing intact. The trade-off is weight: the roof structure has to carry the panels and the ballast.

Mechanically fixed systems. The frames are fixed through the covering into the roof structure, with each fixing properly sealed. This adds less weight but makes holes in the waterproofing, which have to be detailed carefully.

Many installations use a combination. Either way, the mounting has to be designed for the wind on that roof, which is stronger near edges and corners and on taller buildings. For flat roofs we typically use Van der Valk mounting systems, which are designed for this.

South-facing or east-west?

On a flat roof you choose which way the panels face.

South-facing rows give the most output per panel. Each row has to be spaced from the next so the front row doesn’t shade the one behind, especially when the sun is low in winter.

East-west pairs place panels back to back in a low tent shape. They produce a little less per panel, but they need less spacing, so more panels usually fit on the same roof. They also spread generation across the morning and the afternoon.

Our guide to the best direction and angle for solar panels explains the trade-off in more detail.

Planning rules for flat roofs

Most solar on houses is permitted development, which means it doesn’t need a planning application. For flat roofs there are specific limits.

• Height. Panels must not be more than 0.6 metres higher than the highest part of the roof, excluding any chimney.

• Appearance. Equipment should be sited, as far as practicable, to minimise its effect on the external appearance of the building and the area.

• Article 2(3) land. This includes conservation areas. Before installing on a flat roof, you must apply to the council for a decision on whether prior approval is needed for the effect on the building’s appearance.

• Listed buildings and flats. Different rules apply.

Rules change, so we check the position for your address at survey. Our guides to planning permission for solar in London and Article 4 directions explain the detail.

Check the roof first

A flat roof covering, such as felt, rubber (EPDM), fibreglass (GRP) or asphalt, has a shorter life than tiles or slate. The panels will be there for decades, and taking them off later to re-cover the roof means extra cost and disruption.

At survey we look at:

• The age and condition of the covering. Blisters, splits, ponding water and patched repairs are signs it may need replacing first.

• The structure. Joist size, span and condition decide how much weight the roof can carry, which matters most for ballasted systems.

• Drainage. Frames and ballast must not block outlets or stop water draining.

• Access. A rear extension may be reachable only through the house or over a neighbour’s garden. Our guide to whether your London roof is suitable covers scaffolding and access.

If the covering is near the end of its life, it usually makes sense to replace it at the same time. As SPL Roofing, we can re-cover the roof and install the solar as one job, with one scaffold. Our guide to whether you need a new roof before solar explains how to decide.

Rear extensions

The flat roof of a rear extension is often the sunniest and least overlooked roof a London terrace has. A few points are specific to them:

• Size. Extension roofs are often small, so the layout needs to make the most of the space while keeping clear of edges and parapets.

• Shade. The main house and the neighbours’ extensions can shade the roof, especially in winter.

• Windows above. Panels can be visible from upstairs windows, yours and your neighbours’.

• Combining roofs. Panels on an extension can often be combined with panels on the main roof, wired so each part works well. Our guide to adding more panels to an existing system covers extending a system later.

Frequently asked questions

Can you put solar panels on a flat roof?

Yes. Panels are mounted on frames that tilt them towards the sun, held in place by ballast, fixings or both.

Do solar panels damage a flat roof?

They shouldn’t, if the mounting is designed and fitted properly. Ballasted systems avoid holes in the covering; fixed systems need every fixing sealed. The covering should be in good condition first.

Do I need planning permission for solar on a flat roof?

Often not, as long as the panels are no more than 0.6 metres above the highest part of the roof. On article 2(3) land, including conservation areas, you must apply to the council for a prior approval decision first.

What angle are panels on a flat roof?

A low tilt, lower than the ideal for a single panel. That lets rows sit closer together and keeps wind loads down.

Can you put solar on a rear extension?

Yes. Extension roofs are often good for solar, subject to their size, shade and condition.

Request a solar and battery quotation. We’ll check your flat roof’s covering, structure and planning position, and design a layout that suits it.